This invention relates to a process for underground retorting of oil shale.
Researchers have now renewed their efforts to find alternative sources of energy and hydrocarbons in view of recent rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal and tar sands by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usuable gaseous and liquid hydrocarbons.
Vast natural desposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil is contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah, and Wyoming. The remainder is contained in the leaner Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist, and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
Underground formations of oil shale contain various layers, deposits or strata of rich and lean oil shale. The relative richness, leanness, and depth of these layers typically vary throughout the underground formation and depend upon the particular location of the formation.
The process of pyrolyzing the kerogen in oil shale, known as retorting, to form liberated hydrocarbons, can be done in surface retorts in aboveground vessels or in in situ retorts under ground. In situ retorts require less mining and handling than surface retorts.
In vertical in situ retorts, a flame front is passed downward through a bed of rubblized oil shale to liberate shale oil, off gases and residual water. Rich oil shale yields more shale oil and leaves more carbon residue on the retorted shale than lean oil shale. When a sufficient quantity of carbon residue remains on the shale, it provides fuel for the flame front. When insufficient carbon residue exists, as is the case with lean shale, some of the oil produced is not liberated, but is burned to supply the needed fuel.
There are two types of in situ retorts: true in situ retorts and modified in situ retorts. In true in situ retorts, none of the shale is mined, holes are drilled into the formation, and the oil shale is explosively rubblized, if necessary, and then retorted. In modified in situ retorts, some of the oil shale is removed by mining to create a cavity or void space in the retorting area. The cavity provides extra space which is filled with rubble after blasting to provide void space in the bed. The oil shale which has been removed is conveyed to the surface and is available for aboveground retorting.
Over the years various methods for in situ retorting of oil shale have been suggested. Typifying the many methods of in situ retorting are those found in U.S. Pat. Nos. 1,913,935; 1,191,636; 2,481,051; 3,001,776; 3,586,377; 3,434,757; 3,661,423; 3,917,344; 3,951,456; 4,007,963; 4,017,119; 4,036,299; 4,089,375; 4,105,072; 4,117,886; 4,120,355; 4,126,180; 4,133,380; 4,149,752; 4,153,299; 4,158,467; 4,162,808; 4,166,022; 4,185,871; 4,191,251; 4,222,850; 4,194,788; 4,241,952; 4,243,100; 4,263,969; 4,271,904 and 4,285,547; and in an article of the Tenth Oil Shale Symposium Proceedings, at pages 166-178, entitled "Computer Model, for In-Situ Oil Shale Retorting: Effects of Gas Introduced Into the Retort" by R. L. Braun and R. C. Y. Chin of Lawrence Livermore Laboratory, University of California, published by the Colorado School of Mines Press (July 1977). These prior art methods have met with varying degrees of success.
It is therefore desirable to provide an improved process for in situ retorting of oil shale.